Fix <details> spacing

[mxstbr]

This fixes <details> spacing in markdown.

• First, briefly describe your proposal in the title.

<details> in markdown does not have a margin bottom, even though it should have one. Demo:

Note how this text is directly below the details element without the proper bottom margin. I always end up putting a <br> below the details to make it look correct.

Note that this is completely untested. 😅

/cc @primer/ds-core

[mikekavouras]

i have no authority here but good work

[shawnbot]

Thanks for this, @mxstbr! I'd like to include this in the next release after we wrap up #671. 🍻